Every day, more than two million people in the United States are incapacitated by chronic pain (Jessell and Kelly, 1991, “Pain and Analgesia” in PRINCIPLES OF NEURAL SCIENCE, 3rd Ed., (Kandel, Schwartz, and Jessell, ed.), Elsevier, N.Y.). Unfortunately, current treatments for pain are only partially effective, and many of these treatments themselves cause debilitating or dangerous side effects. For example, although non-steroidal anti-inflammatory drugs (“NSAIDs”) such as aspirin, ibuprofen, and indomethacin are moderately effective against inflammatory pain, they are also renal toxins, and high doses tend to cause gastrointestinal irritation, ulceration, bleeding, and mental confusion. Patients treated with opioids also frequently experience confusion, and long-term opioid use is associated with tolerance and dependence. Local anesthetics such as lidocaine and mexiletine simultaneously inhibit pain and cause loss of normal sensation.
Pain is a perception based on signals received from the environment and transmitted and interpreted by the nervous system (for review, see Millan, 1999, Prog. Neurobiol. 57:1-164). Noxious stimuli such as heat and touch cause specialized sensory receptors in the skin to send signals to the central nervous system (“CNS”). This process is called nociception, and the peripheral sensory neurons that mediate it are nociceptors. Depending on the strength of the signal from the nociceptor(s) and the abstraction and elaboration of that signal by the CNS, a person may or may not experience a noxious stimulus as painful. When one's perception of pain is properly calibrated to the intensity of the stimulus, pain serves its intended protective function. However, certain types of tissue damage cause a phenomenon, known as hyperalgesia or pronociception, in which relatively innocuous stimuli are perceived as intensely painful because the person's pain thresholds have been lowered. Both inflammation and nerve damage can induce hyperalgesia. Persons afflicted with inflammatory conditions, such as sunburn, osteoarthritis, colitis, carditis, dermatitis, myositis, neuritis, collagen vascular diseases (which include rheumatoid arthritis and lupus) and the like, often experience enhanced sensations of pain. Similarly, trauma, surgery, amputation, abscess, causalgia, collagen vascular diseases, demyelinating diseases, trigeminal neuralgia, cancer, chronic alcoholism, stroke, thalamic pain syndrome, diabetes, herpes infections, acquired immune deficiency syndrome (“AIDS”), toxins and chemotherapy cause nerve injuries that result in excessive pain.
As the mechanisms by which nociceptors transduce external signals under normal and hyperalgesic conditions become better understood, processes implicated in hyperalgesia can be targeted to inhibit the lowering of the pain threshold and thereby lessen the amount of pain experienced.
Neurotrophic factors have been shown to play significant roles in the transmission of physiologic and pathologic pain. Nerve growth factor (NGF) appears to be particularly important (for review, see McMahon, 1996, Phil. Trans. R. Soc. Lond. 351:431-40; and Apfel, 2000, The Clinical Journal of Pain 16:S7-S11). Both local and systemic administration of NGF have been shown to elicit hyperalgesia and allodynia (Lewin et al., 1994, Eur. J. Neurosci. 6:1903-1912). Intravenous infusion of NGF in humans produces a whole body myalgia while local administration evokes injection site hyperalgesia and allodynia in addition to the systemic effects (Apfel et al., 1998, Neurology 51:695-702). There is also a considerable body of evidence implicating endogenous NGF in conditions in which pain is a prominent feature. For example, NGF is upregulated in dorsal root ganglion (DRG) Schwann cells for at least 2 months following peripheral nerve injury and increased levels have been reported in the joints of animals suffering from a variety of arthritis models (e.g., Aloe et al., 1993, Growth Factors 9:149-155). In humans, NGF levels are elevated in synovial fluid from patients with rheumatoid or other types of arthritis (e.g., Aloe et al., 1992, Arthritis and Rheumatism 35:351-355). Furthermore, it has been demonstrated that antagonism of NGF function prevents hyperalgesia and allodynia in models of neuropathic and chronic inflammatory pain. For example, in animal models of neuropathic pain (e.g. nerve trunk or spinal nerve ligation) systemic injection of neutralizing antibodies to NGF prevents both allodynia and hyperalgesia (Ramer et al., 1999, Eur. J. Neurosci. 11:837-846; and Ro et al., 1999, Pain 79:265-274). Examples of anti-NGF antibodies known in the art include, for example, PCT Publication Nos. WO 01/78698, WO 01/64247, WO 02/096458, and WO 2004/032870; U.S. Pat. Nos. 5,844,092, 5,877,016, and 6,153,189; Hongo et al., 2000, Hybridoma 19:215-227; Hongo et al., 1993, Cell. Mol. Biol. 13:559-568; and GenBank Accession Nos. U39608, U39609, L17078, or L17077.
Clearly, there is a need for new safe and effective treatments for pain, particularly by targeting small molecule mediators or exacerbators of pain such as NGF.